Mills, such as grinding mills, typically include a drum which is loaded with a charge (ores, industrial minerals, rocks, steel grinding media, water, etc.) and is rotated. At start-up, the charge sometimes becomes solidified or locked. Continued rotation of the drum past a certain point may cause the locked charge to drop as a large mass instead of tumbling normally at a lower angle of mill rotation, potentially resulting in severe mechanical damage to the mill.
Some existing grinding mill manufacturers have attempted to address the problems created by locked charge. In one known system, an encoder is attached to a pinion to determine an angular position of the drum. The system systematically aborts a first mill start at a determined angle of rotation. From the mill position after roll back, the system determines whether the charge was locked. In particular, a mill with a locked charge will come back to its original position. Although effective, this grinding mill system requires an aborted start even if the charge is not locked.
Other known grinding mill systems attempt to identify the existence of a locked charge within the drum by either sensing motor torque, by sensing motor current or by sensing noise and vibration produced by a charge cascade. Such grinding mill systems require the use of specific motors or are specific to the characteristics of each mill and the amount and type of charge in the mill.